SAN FRANCISCO VOLCANIC FIELD

NORTHERN ARIZONA

The obsidian sources within this field are, perhaps, the best studied in the Southwest (Jack 1971; Robinson 1913; Sanders et al. 1982; Schreiber and Breed 1971; Shackley 1988). Although the chemical variability within sources in this field is generally less than in the mid-Tertiary sources to the south, the magmatic relationships between some of these sources as previously reported are probably in error (Jack 1971; Sanders et al. 1982).

 

 

 

LandSat Image of the San Francisco volcanic field

 

 

 

 

 

 

The coalesced rhyolite domes known as Government Mountain (view from the south)

Government Mountain. Sections 24, 25 R4E, T22N and Sections 19, 20, 29, 30, R5E, T22N USGS Parks 7.5' Quad, Kaibab National Forest, south central Coconino County, Arizona (updated 6.25.98). This is, as noted earlier, perhaps the best known contemporary obsidian source in Arizona. Shackley (1986b), however, showed that RS Hill glass may be equally common in Preclassic Hohokam contexts in central Arizona. Government Mountain is a single rhyolite dome that exhibits rhyolite/tuff/obsidian within the alluvium around the base of the dome structure. Even after years of modern and prehistoric collection, nodules up to 15 cm in diameter are common. Most nodules exhibit little cortex, per se, but some are covered with ash from the alluvium or exhibit a thin gray or brown weathered surface. The glass is aphyric, but the fabric is megascopically granular and distinctive, apparently from microphenocrysts of alkali feldspar (Wolfe et al. 1987:13).  Some lighter gray and more vitreous material was recovered on the south slope. Thin flakes are cloudy gray when viewed with transmitted light. Secondary deposition occurs only within a short distance (2 km) around the base and is most noticeable south into Government Prairie.

Wolfe et al. (1987) place the date by K-Ar at 2.10±0.03 mya.

Reduced cobbles and flakes are very abundant and densities of cultural obsidian reach 200 per 5m² in places, especially on the west and south slopes; some of this, however, is modern reduction from local knappers. Bifacial core preforms also occur sporadically on the dome, and most are broken (usually lateral snap-fractures). No other artifactual material was observed. Published references are the same as for the previous San Francisco sources, although Robinson only mentions it in passing (1913:67).

Update: In 2004 I sampled the dome complex toward the top of the domes and on the north side. Analysis of many archaeological data from this source suggested lower Rb values and higher Zr values than the original 10 source standards.  At the base of the north side of the complex a number of banded nodules were located.  Some of these exhibited the lower Rb values evident in the archaeological record.  The table below reflect the mean and central tendency for the original 10 source standards and 14 additional samples from near the top of the dome complex and the north side.

Suggest:

Wolfe, E.W., Ulrich G.E., Holm, R.F., Moore, R.B., and Newhall, C.G. (1987).  Geologic map of the central part of the San Francisco Volcanic Field, north-central Arizona. USGS Miscellaneous Field Studies Map MF-1959.

Elemental concentrations for Government Mountain source standards*. All measurements in parts per million (ppm).  

 
* Cow Canyon and Government Mountain exhibit similar chemistry in a number of elements. They are megascopically distinctive and Ti, Zr, Nb, and Ba effectively separate these two sources.

Sitgreaves Mountain. Sections 7,13,14 R3E, T23N USGS Williams 15' Quad; Sections 16,17,19,20,24,25,31,32,36 R4E, T22N USGS Moritz Ridge and Parks 7.5' Quads. Kaibab National Forest, south central Coconino County, Arizona. Sitgreaves Mountain (or Peak) is the largest single mass of rhyolite in the San Francisco field (Robinson 1913). Abundant eroded cobbles of obsidian are present around the flanks of this mountain. Some of the cobbles are up to 20 cm in diameter. The nodules are part of a rhyolite/tuff/obsidian alluvium and densities up to 10 per m² were recorded. No obsidian was recorded 'in situ' or in large blocks toward the peak. There appears to be no appreciable secondary deposition, although the eroded material in Spring Valley is chemically identical to both Sitgreaves and RS Hill. The rounded nodules exhibit a tan to brown cortex and a lustrous, but somewhat micro-granular interior. The glass is slightly vitrophyric with uncommon alkali-feldspar phenocrysts up to 3 mm in diameter. The glass is near opaque and gray with thin flakes exhibiting a cloudy gray pattern. This material, while similar to RS Hill and Government Mountain obsidian does not reduce as efficiently and the large sparse phenocrysts hamper control.

The nodules were frequently reduced at the source and flakes occur throughout the source area, but no intensive reduction areas were located. Published sources include Jack (1971); Moore et al. (1960); Robinson (1913); Sanders et al. (1982); Schreiber and Breed (1971).

Suggest:

Wolfe, E.W., Ulrich G.E., Holm, R.F., Moore, R.B., and Newhall, C.G. (1987).  Geologic map of the central part of the San Francisco Volcanic        Field, north-central Arizona. USGS Miscellaneous Field Studies Map MF-1959.

Raw elemental concentrations for Sitgreaves Mountain (SM) and RS Hill (RS) source standards*. All measurements in parts per million (ppm).

RS Hill. Sections 15, 16, 21, 22, 28, 27 R4E, T22N USGS Parks 7.5' Quad, Kaibab National Forest, south central Coconino County, Arizona. This is a relatively small rhyolite dome that produced the largest nodules recovered in this study (35 cm in diameter). The nodules are similar to those from Sitgreaves, but not as eroded and therefore generally more angular. Gray cortex predominates. The fabric is also megascopically vitrophyric and micro-granular with sparse alkali-feldspar phenocrysts up to 3 mm in diameter. The glass however, is frequently blacker, more vitreous and a higher quality knapping obsidian. The fabric looks megascopically very similar to Government Mountain, but is not entirely aphyric. Secondary deposition occurs as slope wash into the Spring Valley Wash system, but the material is only transported a few kilometers south. Reduction of the nodules is very common and flake densities approach 100 per 5m² in places. Published references are the same as for Sitgreaves.

Kendrick Peak. Sections 1,2,3 R5E, T23N; Sections 35,36 R5E, T24N USGS Kendrick Peak 7.5' Quad, Kaibab and Coconino National Forests, south central Coconino County, Arizona. Kendrick Peak is a large composite cone consisting of five lavas (Robinson 1913). The rhyolite/obsidian is located on the northeast and east slopes. Robinson describes a "thick flow of black, lustrous obsidian" on the northeast divide (1913:55). This was not relocated on the survey, but abundant nodules up to 10 cm in diameter were collected from the east and northeast slopes near Crowley Park. Nodule densities up to 50 per 5 m² were recorded especially toward the northeast divide. Secondary deposition occurs down into the Crowley Park meadow 5 km east.

Cortex is generally absent with just gray-black weathered glass. The glass is vitrophyric with abundant feldspar phenocrysts up to 2 mm in diameter. The color is gray-black and nearly opaque. Thin flakes show the distinctive cloudy gray translucent character common in San Francisco obsidians.

The vitrophyric and nearly devitrified fabric in most specimens hampers control during reduction. Kendrick is not a good raw material for tool production, and was not detected in any sites in this study. Interestingly, prehistoric reduction of nodules was common, occurring everywhere on the slopes. Artifactual density was difficult to estimate in the heavy pine duff. Published references are the same as previous San Francisco obsidians.

Note: Elemental concentrations for Kendrick Peak, San Francisco Peak, and O'Leary Peak are not presented here due to their rarity in archaeological context. See Shackley (1995) for these data.

Slate Mountain (Wallace Tank). Southeast corner of Section 2 R5E, T24N USGS Kendrick Peak 7.5' Quad, Coconino National Forest, south central Coconino County, Arizona. Jack (1971) and Schreiber and Breed (1971) both identify Slate Mountain as the source of obsidian here, but it is actually derived from a small rhyolite/obsidian dome at the base of the south slope at Wallace Tank. The dome measures a mere 100 meters in diameter by 10 meters high; the structure, however, is practically entirely composed of equal proportions of black and mahogany-red and black nodules. A vertical adit dug into the top of the dome to a depth of 2 meters exhibits a series of alternating layers of rhyolite and reddish perlite that contains mahogany-red and black nodules up to 10 cm in diameter. The nodules are as dense as 10 per m² on the surface of the dome. The cortex and fabric are similar to Kendrick, but the glass is more vitreous, the phenocrysts are smaller (<2mm) and less dense, and half the nodules are a mixed mahogany-red and black. Secondary deposition does not occur more than a kilometer away from the dome structure.

Prehistoric reduction of the nodules is very common (up to 20-50 flakes and cores per m²) and there appears to be no preference for the reddish or black nodules, although subsurface deposits suggest that the red material may have been more common at one time. One small plainware sherd was recovered as well as two non-Slate Mountain obsidian artifacts: A unifacially retouched flake made from Government Mountain material and a biface distal end fragment made from RS Hill material. Published references include all the above for San Francisco sources in addition to Mintz (1942).

SLATE MOUNTAIN (WALLACE TANK) 

Element      Mean    Std Dev   Minimum   Maximum       N   

Ti          649.86      72.71     553.2     773.9     10

Mn          446.30      24.69     424.8     501.0     10

Fe        11617.40     450.53   11178.6   12643.1     10

Rb          115.26       3.91     109.9     122.9     10

Sr           67.38      10.07      61.1      94.8     10

Y            21.90       1.89      18.2      24.7     10

Zr          143.16       4.49     134.5     149.4     10

Nb           45.84       2.51      40.6      48.9     10

Ba          753.62      24.94     716.9     792.4      9

San Francisco Peaks (Fremont-Agassiz Saddle). Northeast corner of Section 5 R6E, T22N USGS Humphrey's Peak 7.5' Quad, Coconino National Forest, south central Coconino County, Arizona. This source is located above 3300 m in elevation on the northwest slope of Fremont Peak near the saddle. The obsidian, located within a rhyolite lava that measures less than 20 m across, occurs as a large vitrophyric block about 5 meters in diameter with abundant eroding nodules up to 15 cm in diameter. The cortex is variable from ashy-rhyolite to weathered glass, and the glass itself is extremely vitrophyric with abundant alkali-feldspar phenocrysts up to 4 mm in diameter with accessory zircon and rare earth titanosilicates (see Burton 1986). Many nodules are interbanded with ash. The color is gray-black and translucent around the margins. The material will not allow controlled fractures and is generally useless for biface manufacture, although flakes can be removed on the more vitreous materials. This source was not apparent in any of the sites in this study.

There appears to be some prehistoric reduction here, but the density is light and many 'flakes' could have been produced by freeze-thaw processes. Published references include Jack (1971), Moore et al. 1960, Robinson (1913), and Schreiber and Breed (1971).

O'Leary Peak/Robinson Crater. Section 10 R8E, T23N USGS O'Leary Peak 7.5' Quad, Coconino National Forest, south central Coconino County, Arizona (updated 6.25.98). This is a low density, low quality rhyodacite glass source on the eastern edge of the field. The nodules, up to 12 cm in diameter, were collected between O'Leary Peak and Robinson Crater. Rhyodacite units (Moore and Wolfe's Qoo unite) occur on the slopes of O'Leary and are surrounded by Sunset basalt cinders (see Moore and Wolfe 1976). The obsidian occurs sporadically in the cinders and rhyodacite, and the density is generally less than 1 per 5m². The obsidian is highly vitrophyric (pitchstone). Most looks like a grey-black glassy rhyodacite. Preferred planes of cleavage are caused by bands of ash in many nodules, and the cleavage planes and phenocrysts do not allow successful knapping other than sporadic flake removals. No O'Leary Peak material was located in any of the sites in this study, but artifacts have been reported in the Sunset Crater area.

Interestingly, reduced cores and flakes do occur (less than 1 per 5m²) and two plainware and one black-on-white sherd were recorded. Additional published sources include Jack (1971), Moore et al. (1960), Schreiber and Breed (1971).

Suggest:

Moore, R.B., and Wolfe, E.W. (1976).   Geological map of the eastern San Francisco volcanic field, Arizona.   Miscellaneous Investigations Series, Map I-953.  USGS.

Wolfe, E.W., Ulrich G.E., Holm, R.F., Moore, R.B., and Newhall, C.G. (1987).  Geologic map of the central part of the San Francisco Volcanic Field, north-central Arizona. USGS Miscellaneous Field Studies Map MF-1959.

This page maintained by Steve Shackley (shackley@berkeley.edu).
Copyright © 2002 M. Steven Shackley. All rights reserved.
Revised: 11 December 2009

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